Method of forming honeycomb

ABSTRACT

A method of stretch forming arcuate lightweight, rigid, honeycomb-type panels by providing uniform controlled elongation of both skins of the panel being formed instead of stretching the outer skin while compacting the inner skin in forming the curved surface.

Chester et al.

1111 3,788,117 [451 Jan. 29, 1974 METHOD OF FORMING HONEYCOMB [56] References Cited [75] Inventors: Frank S. Chester, Laguna Beach; UNITED STATES PATENTS Claude Johnsml; Keith 3,060,561 10/1962 Watter 29/455 Kavanagh, both of Fullerton; 2,824,594 2/1958 Gray 72/297 Leonard R. Van Horik, Long Beach, 3,262,301 7/1966 Langworthy 72/297 7 all of Calif. [73] Assignee: McDonnell Douglas Corporation, i i'L QD2E FQL L'EQ'Tl PE hA- SQFLHWH Santa Monica, Calif Attorney, Agent, or -Fzrm-Robert 0. Richardson, Walter J. Jason, and Donald L. Royer [22] Filed: June 19, 1972 21 Appl; 190.; 263,861 57] ABSTRACT" A method of stretch forming arcuate lightweight, [52] US. Cl 72/296, 29/455 LM, 72/379 rigid, honeycomb-type panels by providing uniform [51 int. Cl B2ld 11/02, B2ld 47/00 controlled elongation of both skins of the panel being [58] Field of Search 72/295, 296, 297, 379, 392; formed instead of stretching the outer skin while com- 1 13/1 16 A; 29/455 LM; 264/291, 292 pacting the inner skin in forming the curved surface.

' 6 Claims, 9 Drawing Figures /22 1 i i I'H I I "II llllllllllllllllllfi 1 1 1111-111 my ml ill IIIUHE I i I I I 11] l I :iii //0 Mi PATENTEDJAI 29 I974 SHEET 3 (IF 3 METHOD OF FORMING HONEYCOMB BACKGROUND OF THE PRESENT INVENTION Noise pollution is an important consideration of aerospace companies involved in the manufacture of large jet engine aircraft. One solution to a noise suppression problem in the engine inlet is to use a lightweight, rigid, honeycomb-type panel consisting of stainless steel skins and a very thin (0.0025 inch) foil core, also of stainless steel. The core walls are resistance-welded' together and to the skins, the foil being slightly crimped at both ends to afford a more substantial weld to the skins. At least one of the skins has a plurality of perforations for airflow communication with the inner cells where sound suppression is achieved. These panels are usually supplied in planar or flat sheet form and the problem is to bend these sheets into arcuate and irregular configurations. The problem presented, and which is solved by the present invention, is in the shaping of the panels without bending, buckling or other damage to the foil core'walls. It has been found that when such a damaged panel is subjected to the airstream, high decibel vibrations and the tremendous pressures of the airstream will cause the damaged walls to disintegrate, compounding the damage and rendering the panel ineffective as a noise suppression device. The forming of the panel in one operation without a springback or requiring rework is a further problem.

SUMMARY OF THE PRESENT INVENTION The above problems are overcome through the provision of a uniform control elongation of the panel as it is stretched around a die in such manner that both surfaces of the panel are stretched without applying compression forces to the inner curved surface. Each minute increment of panel is stretched by the same amount as all other increments in order not to damage the foil core. The uniform positive elongation of both skins of the panel being formed is done througha wire-actuated control operating off a template cut to the form block contour. During the stretch operation, the control maintainsthe same amount of pre-stretch, dimensionally and proportionally, as was setinto the system initially. In accomplishing these uniform stretching operations, the panel is made slightly Vshaped so that the panel width at its mid-portion is slightly less than at its ends. This smaller Coke bottle-shaped portion engages the die first as the metal is stretched and wrapped.

around, and, accordingly, is the first increment of panel to be stretched. As it is stretched, it work-hardens and becomes less stretchable while the next outward increment of the panel, as it contacts the die, is then stretched. This progression occurs successively and progressively throughout the entire movement of the panel around the die until the stretch forming operation is completed. During the stretching operation, the operator uses a simple but direct method of checking elongation. A machinist scale is taped by one end to the panel and the short scale or metal strip is taped over its other end. As the outer skin stretches, the scale moves with relation to the edge of the strip, providing a reading that may be interpreted into percent elongation by standard tables provided. A visual check is provided afterwards by observing the elongated holes on the skins.

BRIEF DESCRIPTION OF THE DRAWINGS the ribbon direction;

'FIG. 3 is a perspectiveview showing the curvature of a honeycomb when it is bent in a direction transverse to the ribbon direction;

FIG. 4 is a schematic illustration of the tension and compression forces in a workpiece during conventional stretch forming;

7 FIG. 5 is a schematic illustration of the tension forces in a workpiece subjected to stretch forming with elongation control;

FIG. 6 is a schematic illustration of stretch forming when applying the principles of elongation control;

FIG. 7 is an elevational view of astretch press and a honeycomb panel being subjected to elongation control forming; I

FIG. 8 is a perspective view partly in section showing the location of some of the panels in typical use; and

FIG. 9 is an elevational view illustrating where stretched panels may be used on a jet engine for noise suppression.

DETAILED DESCRIPTION OF THE DRAWINGS Reference is now made to FIG. 1 wherein there is shown a perforated panel 10 of the type commercially available. It includes a backing sheet 12 with a plurality of perforations l4 and a face sheet 16. Between these sheets is a honeycomb core 18 which consists of a plurality of corrugated strips 20 welded together at spaced points to provide walls for cavities between the two sheets. These walls are very thin and individually are easily crushable but when in the composite such as shown it becomes a strong, lightweight panel having noise suppression properties to an airflow passing the perforations 14. Such panels are available in planar flat form and in such condition present no problems of the type encountered when such a panel is bent to a circular or other curvature.

FIG. 2 and FIG. 3 are pictorial presentations of the reaction of honeycomb to bending forces both in the ribbon direction and transverse to the ribbon direction. The ribbon direction is defined as the direction of the individual strips which are welded together at spaced points to form the honeycomb cell walls. Thus, in FIG. 2 a plurality of strips 20 extend transversely of the panel 18 are welded together at spaced points 22 to form the open cell core. In this configuration the ribbon direction is that direction of a double-pointed arrow 24. In the configuration shown when a downward pressure is exerted on the sides 26, 28 as shown by the arrows 30, 32, and an upward pressure is exerted in the middle 34, as shown by arrow 36, bending occurs in the ribbon direction as shown by the double-headed curved arrow 24. A problem occurs that does not occur with an ordinary sheet of sheet metal. The ends 38 and 40 curl up in the direction of arrows 42, 44. Moreover,

shown in FIG. 1, compression and tension forces would occur that, in extreme cases, might cause the cell walls to buckle and result in a damaged panel as previously discussed. In any event, the tension and compression forces result in an erratic neutral force axis within the core which will result in warping, springback and inaccurate bending of the panel. This requires reworking, a second bending effort or the panel may be damaged beyond repair.

FIG. 3 shows the bending of the core in a direction normal to that of the ribbon direction. In this case the transverse edges 38, 40 are urged downwardly in the direction of arrows 52, 54. An upward force is applied in the middle in the direction of arrow 56. This results in a longitudinal bending as shown by the curved double-headed arrow 58. However, here again the core material reacts unlike that of an integral sheet of sheet metal, in that the corners 46, 48 transverse ends 38, 40 curl upwardly in the direction of arrows 58, 60. Here again, when the honeycomb core is welded to face sheets, unwanted tension and compression forces caused by the anticlastic bending effect of the composite panel tends to cause the cell walls to bend and create unsuitable damage, and at best to cause warpage or springback and require reworking. It is the problems pictorially represented in FIGS. 2 and 3 that are solved by the present invention.

The solution to the foregoing problems and a description of the present invention can best be understood following a brief discussion of conventional stretch forming of ordinary sheet metal as pictorially and schematically shown in FIG. 4 and the treatment of the same sheet of metal with elongation control as set forth in FIG. 5. Conventional stretch forming, although superior to roll-forming, requires rework due to the warpage, springback, and distortion that occurs during forming as a result of non-uniform elongation. In this case, unlike the roll-forming situation, the neutral axis fluctuates erratically throughout the thickness of the part, depending upon the operators skill in controlling the tension cylinder rate relative to the wrapping cycle. Variables in cross section and part length (most elongation occurs near the ends of a part) also influence the location of the-neutral axis. As shown in FIG. 4, a sheet metal part 62 is stretched and curved in an arcuate manner. As shown in cross section, there are compression forces 64 on the inner portion and tension forces 66 on the outer portion with a neutral axis 68 fluctuating erratically throughout the thickness of the part. A conventional stretch forming of a honeycomb composite panel such as shown in FIG. 1, in extreme cases, could thus subjected to high velocity airstreams. At best, the inner skin would wrinkle, springback or warpage will occur and a second bending or rework of the panel would be required.

In contrast to the tension compression forces generating a fluctuating neutral axis within the workpiece, as illustrated in FIG. 4, a stretch forming technique for sheet metal has been developed with elongation control. Application of this elongation control process establishes and maintains the neutral axis 70 in FIG. 5 at a specified distance outside the inner surface 72 of the material 74 to be formed. Simply, this means that both surfaces 72 and 76 will be stretched. Although the inner surface 72 is stretched to a lesser degree, the total effect is a positive stretch for both sides (rather than stretching the outer surface and compressing the inner surface), thus permitting the material to be formed without springback or distortion.

The operation is essentially automatic and precludes the necessity for the operator to utilize judgment, experience, or intuition. The elongation control equipment is also completely unaffected by such considerations, or by material inconsistencies or pressure fluctuations.

One method of stretching both sides of a sheet of metal, to eliminate the area of compression in the metal when it is bent, is disclosed in US. Pat. No. 2,824,594, issuing Feb. 25, I958 to L. R. Gray. It relates to a machine having a template fixed to the die and a stretch controlled tape extending from a work clamp to the template" and reference is herein made to this patent for a full and complete description of the apparatus used for this purpose.

The principles of elongation control are explained briefly with reference to FIG. 6 of the present application. Here there is shown a die 78 having a desired arcuate configuration shown by surface 80, against which the work part 82 is to be bent, and at the same time, stretched, by jaws 84, 86. Spaced inwardly from surface is a template 88 having a similar surface configuration 90. A sensing wire 92 of a constant length is stretched between two sensing devices 94, 96 which are spaced from the jaws 84, 86 by the same amount as the setback of template 88 from the surface 80 of die 78. In theory, as well as in practice, the jaws 84 and 86 bend the workpiece 82 around the die 78 to the position shown in phanthom lines in which like parts have like numerals followed by the letter A. The wire 92 also wraps around template 88 as the sensing devices 94, 96 are moved to their new position. Since the panel 82 follows curvature 80 while the wire 92 follows curvature 90 but in an inward spaced relationship, in order for the jaws 84, 86 to maintain the spaced relationship with the sensing devices 94, 96, i.e., remain aligned normal to the wire 92 at that point, these jaws must stretch apart and thus elongate the panel 82. Such elongation is represented by the double-headed arrow B to indicate the amount of elongation when the panel 82 has reached its phantom position 82A. Typically, this elongation is about a one percent stretch. Thus, if a 60 inch radius is designed for die surface 80, then the amount of setback is 0.6 inch.

FIG. 7 is an elevational view of a machine designed to carry out the principles in the aforementioned Gray patent and which is adapted to the method practiced in the present invention. Here there is shown in elevational view the die surface 80 on the top of which is mounted the setback template surface 90. A honeycomb panel workpiece 98 has attached to it some ears 100 of sheet metal which are gripped by jaws 102 of the machine which will stretch the panel 98 in that direction of arrows 104 as it is wrapped around die surface 80. An important criteria is that the ears do not stretch or expand when the panel is stretched. These jaws 102 are hydraulically controlled and are mounted on a pair of arms 106 which pivot at pivot points 108 as they pivot rearwardly in the direction of arrows 110. The outer portion of these arms 106 are mounted on rollers 112 which move along the floor 114.

Although the honeycomb sheets from the commercial fabricator are quite expensive and are priced by the square foot, the sheets contain a margin of waste for which there is no charge. Thus, in FIG. 7, the area within dashed line 116 on panel 98, is an area of acceptable panel for which the fabricator makes a charge and the area bonded to the ears 100 and longitudinally above and below the dashed line area is waste. Therefore, in accordance with the present invention, arcuate portions may be cut from the waste without unduly adding to the cost. Hence, panel 98 has an arcuate upper edge 118 and an arcuate lower edge 120 in order that a reduced panel widthg122 appears at its midpoint in length. This part is the first increment to engage die 80 and to be subjected to stretch. This reduced width makes this area of the panel weaker than other areas and is the first to expand during the stretch and bend operation. The stretching then work-hardens the area and this, plus the frictional contact with the die, stops the stretching in that area while the minute increments next adjacent will stretch. As previously explained, these increments of stretching successively, continuously and smoothly expand along the panel to the jaws as the panel is wrapped over the die. While this arcuate reduced width, or Coke bottle configuration, is vital to the fabrication of certain configurations and for certain thicknesses, a dog-bone technique is acceptable in other. This dog-bone technique is simple having the ends of greater width than the midportion to prevent the ends from stretching and to provide an adequate gripping surface for the stretching jaws. When the panel is properly shaped the holes 14 in panel in FIG. 1 will have a uniform elongated or oval appearance which provides for a very convenient visual inspection.

In FIG. 8 there is shown an examplary use of the panels in the inlet duct to the tail mounted engine ofa trijet aircraft. Here there is shown the engine 124 mounted rearwardly of support spars 126, 128. Between the spars are a plurality of arcuate panels fastened to the spars by bolts or rivets 132 to form an annular air duct enclosure. Elongated holes 134 on the inner surface 136 provide communication of the engine inlet airflow with the honeycomb cells within the panels. A similar ring of panels 138 extends downstream and is attached to spar 128. A bellmouth ring of panels 140 is attached to the inlet of engine 124 and fits into the end of ring panels 138.'The overlapping arrangement requires a varied thickness in the panel edges which in turn requires a compound die in their fabrication.

In FIG. 9 there is shown an aircraft engine 142 of the type adapted for wing or fuselage mounting on commercial aircraft. Various acoustic panels are placed in the inlet and exhaust ducts to suppress the noise of airflow passing therethrough. Here the inlet 144 and exhaust section 146 are elongated more than in previous engines. The inner surface 148 of the inlet 144 consists of honeycomb panels with perforations on the inner surface. Not only is it annular in front view but the configuration is ,arcuate or barrel-shaped in side cross section as shown. The bullet 150 uses a cylindrical honeycomb core panel 152 as its outer wall. Between the bullet 150 and the inlet inner surface 148 is an inner ring 154 and an outer ring 156 held in place by a strut assembly 158. Both rings consist of honeycomb panels appropriately curved in front view and in section. Since both sides of these rings are subjected to airflow, their surface walls are perforated on both sides of the honeycomb. Similarly, the walls 160 of the primary nozzle, walls 162 of the secondary nozzle, and augmenter housing 164 consist of honeycomb panels with compound curves.

Having thus described an illustrative embodiment of the present invention, it is to be understood that modifications thereof will become apparent to those skilled in the art and it is to be understood that these deviations are to be construed as part of the present invention.

We claim:

1. A method of forming honeycomb panels comprising the steps of:

placing a honeycomb panel against a die, and

uniformly and continuously stretching the panel commencing at its midportion while concurrently wrapping the panel over the die.

2. The method of forming honeycomb panels as in claim 1 wherein said panel is first cut to a configuration to cause sequential and continuous stretching from its midportion to its ends.

3. The method of forming honeycomb panels as in claim 2 wherein the midportion has a lesser width than the ends of the panel.

4. The method of forming honeycomb panels as in claim 1 wherein both sides of the panel are subjected to stretching to place the compression-tension forces neutral axis exteriorly of the panel.

5. The method of forming honeycomb panels as in claim 1 wherein annular openings are made in at least one side of the panel before it is stretched and then the shape of the resulting elongated openings are visually inspected after the panel is stretched to confirm the uniform elongation thereof.

6. The method of forming honeycomb panels as in claim 2 wherein the panel has a waste portion border and the cut is a gradual Vcut to make the midportion of the panel of less width than its ends.

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MCQY- MQGiBsoM-JRF f 1 c. MARSHALL DANN Attesting OffiiCgn x v Commissioner of Patents 

1. A method of forming honeycomb panels comprising the steps of: placing a honeycomb panel against a die, and uniformly and continuously stretching the panel commencing at its midportion while concurrently wrapping the panel over the die.
 2. The method of forming honeycomb panels as in claim 1 wherein said panel is first cut to a configuration to cause sequential and continuous stretching from its midportion to its ends.
 3. The method of forming honeycomb panels as in claim 2 wherein the midportion has a lesser width than the ends of the panel.
 4. The method of forming honeycomb panels as in claim 1 wherein both sides of the panel are subjected to stretching to place the compression-tension forces neutral axis exteriorly of the panel.
 5. The method of forming honeycomb panels as in claim 1 wherein annular openings are made in at least one side of the panel before it is stretched and then the shape of the resulting elongated openings are visually inspected after the panel is stretched to confirm the uniform elongation thereof.
 6. The method of forming honeycomb panels as in claim 2 wherein the panel has a waste portion border and the cut is a gradual V-cut to make the midportion of the panel of less width than its ends. 